1. Field of the Invention
The present invention relates to a method of forming fine grain equiaxed castings from molten metals.
2. Discussion of the Prior Art
Early wrought superalloys were produced by conventional ingot and hot working technologies. The need for improved properties, primarily in the aerospace propulsion industry, eventually led to the development of more highly alloyed materials which became increasingly difficult to produce in large sizes without significant chemical and microstructural segregation, particularly along the ingot centerline where the metal freezes last. This undesirable condition not only affected forgeability, but also affected the resultant properties of the forgings containing this type of structure.
A conventionally produced casting contains a combination of columnar and coarse equiaxed grains and the resulting grain size of a casting generally is larger as the size of the casting increases. This increases the forces required to forge the material and also the tendency for cracking during hot working operations.
A solution to these problems was the successful adaptation of powder metallurgy approaches to the manufacture of uniform grained and chemically homogeneous products which responded well to forging practice. Furthermore, it developed that such fine grained materials (e.g., ASTM 10-12) were superplastic when deformed at preferred temperatures and strain rates which enabled the production of very near net shapes with relatively modest deformation forces. The fine grain size improves overall forgeability and allows the utilization of isothermal forging procedures. While the latter operation is slow and ties up high capital cost equipment, it has the ability to produce products nearly to final shape and thus avoid the waste and associated machining costs attendant with the removal of excess stock.
The production from metal powders, however, is not without technical shortcomings, especially with respect to superalloys. Superalloy powders usually are produced by atomization in an inert atmosphere and subsequent screening to remove all but the preferred particle sizes. As cleanliness demands have increased, more of the coarser particle fractions are discarded to satisfy this requirement. Typically, 60% yields are expected for the process and this represents a significant premium cost factor for the product. This has inhibited widespread use of such materials where cost is a significant factor.
In addition, superalloy powder metallurgy products are susceptible to quality related problems which can reduce substantially the mechanical properties of the product. These include boundary conditions related to the original powder surface and thermally induced porosity resulting from trapped atomizing and handling gas (e.g., argon). Process controls necessary to avoid these problems can present a substantial expense. Thus, if a casting process could be developed which produces a chemically homogeneous, fine grained add sound product, an alternative to the powder metallurgy process might be realized with lower manufacturing cost.
As noted above, the finer grain, size of the article produced, the better is its forgeability and the associated economics of production are enhanced. Investment castings usually benefit by having the finest possible grains to produce a more uniform product and improved properties, thus it is conventional to control and refine the grain size of the casting through the use of nucleants on the interior surface of the mold. While this produces a degree of grain refinement, the effect is substantially two dimensional and the grains usually are elongated in the direction normal to the mold-metal interface. This condition also occurs without a nucleant where metallic ingot molds are used. In either instance combined use of low metal superheat and low mold temperature, both at the time of pouring, are means by which the grain size can be refined; however, the resultant microstructure remains dendritic and characteristic of traditional foundry processing. The most desirable microstructure would be, in addition to minimum grain size, the presence of a cellular, or nondendritic, structure to facilitate thermal processing procedures. Such a microstructure would result from a high nucleation and freezing rate of the molten metal at the time of casting. Means for achieving this product are described in U.S. Pat. Nos. 3,847,205, 3,920,062 and 4,261,412. Using the techniques disclosed in these references, grain sizes of ASTM 3-5 can be readily achieved.
Other techniques have been employed to refine grain size in both investment casting and ingot manufacture which include the addition of finely distributed solid particles within the melt as nucleation sites. This has found little favor with superalloy users because of undesired compositional changes or the possibility that residual foreign material may provide sites at which premature failure may initiate. Alternatively, the molten alloy may be stirred mechanically, such as in rheocasting, to refine its grain size. This often results in a nondendritic structure containing two components--closely spaced islands of solid surrounded by a matrix of material which remains liquid when the mixing is discontinued--which usually occurs when viscosity increases abruptly at about 50% solidification. This process works well with lower melting point materials. It has not been successful on a commercial scale with superalloys due to their high melting point and the fact that the ceramic paddles or agitators are a source of potential contamination of the melt in the ingot manufacturing process. Reductions of fluidity would preclude the application of rheocasting to the investment casting process.
A more desirable method involves the seeding of the melt as described in U.S. Pat. No. 3,662,810. A related technique, described in U.S. Pat. No. 3,669,180 employs the principle of cooling the alloy to the freezing point to allow nuclei to form, followed by reheating slightly just before the casting operation. If in doing this isolated grains nucleate and grow dendritically in the melt, they may not fully remelt upon reheating thus producing random coarser grains in the final product. Both procedures work but require sophisticated control procedures. In addition, neither address the problem of alloy cleanliness, or inclusion content. This requirement has grown in importance as metallurgical state-of-the-art improvements are made and product design limits are advanced.
Whether casting in an ingot mold or an investment shell it is normal to see a characteristic array of grain structures from the surface to the core of a casting. Adjacent to the surface it is customary to observe a chill zone which usually is nondendritic in nature. Immediately below this zone are columnar dendritic grains lying normal to the surface and parallel to heat flow. One would expect to find a coarse dendritic equiaxed structure below the columnar zone contrary to that observed by this casting practice. The aforementioned columnar condition is unsatisfactory in an investment casting and must be removed by machining or other means from an ingot surface before forging operations are initiated. Failure to do this will cause premature cracking during forging reductions.
It is, therefore, an object of the invention to provide a method for the casting of cellular fine grained ingots, forging preforms and investment castings in which the above disadvantages of the prior art may be obviated.
Specifically, it is an object of the invention to provide a casting having a desired microstructure.
It is an additional object of the invention to form such castings using equipment that can be used on a commercial scale.
It is a further object of the invention to provide castings having little or no surface connected porosity such that hipping of the casting can be successfully employed to eliminate any casting porosity.
Other objects and advantages of the invention may be set out in the description that follows, may be apparent therefrom or may be learned by practice of the invention.